The present invention relates to an apparatus and method for the continuous countercurrent contacting of particulate solids with fluids, and more particularly to the washing of salt from alumina microspheres.
Countercurrent solid-fluid contacting may be used for washing, leaching, absorption, ion exchange or heat transfer. Of particular interest herein is the washing of the copious quantities of salt from alumina microspheres produced by sol-gel technology. The salt must be removed so that the microspheres can be used in heat transfer applications.
Economic and ecological considerations often dictate that a contactor should achieve its objectives with minimal overall fluid flow. Where the objective is to recover cleansed particles, minimal flow translates to less fresh fluid being spent and less spent fluid having to be disposed of. In cases where it is desired that the foreign material be recovered, recovery may be facilitated by the greater concentration of material in a more manageable volume of fluid.
One factor in achieving the efficient use of fluid is the uniformity among the particles in duration of contact with the fluid. Where the duration is not uniform, additional fluid is necessary to ensure thorough cleaning.
Another factor leading to such efficiency is the degree to which partially spent fluid is utilized advantageously. Partially spent fluid may be effective at washing highly contaminated particles, but ineffective at washing relatively clean particles. This principle is employed in countercurrent multistage apparatus where fresh fluid is introduced at the last stage visited by flowing particles and the eventually spent fluid is evacuated at the first stage visited by the particles.
The solid particles must remain fluidized for the contacting to be effective. This imposes a constraint on many contactors that the overall fluid flow rate be above the minimum fluidization velocity of the particles. However, in many applications this minimum overall flow rate will result in an excessive amount of wash liquid required or an insufficient concentration of leach out in the wash fluid.
This inefficiency can be mitigated in designs where the overall flow and the fluidizing flow can be varied independently. U.S. Pat. No. 2,632,720 discloses an apparatus in which the fluidizing flow can be varied independently; however, it may not be raised above the overall flow. Any attempt to increase the fluidizing flow above the through flow will cause liquid down flow along with the solids and result in total liquid mixing in the contactor which would lead to a complete negation of any benefits to be derived from multistaged processing.
A restricted base opening (RBO) design, which is disclosed in U.S. Pat. No. 4,062,697 incorporated herein as though quoted in its entirety, does permit the fluidizing flow in each stage to be varied above the overall flow rate. However, a number of features of the RBO design affect its usefulness and efficiency.
One problem is that particle flow in an RBO device is very sensitive to the fluidizing height of the stage, the recirculation flow and the rate of particles into the stage. As a result of this sensitivity, the RBO contactor must be constantly monitored to maintain effective operation.
Additionally, in the RBO design some particles receive inadequate washing. Statistically, all particles on a stage have the same probability of passing to the next stage during any time period. This means that particles that have just reached the stage have the same probability of passing to the next stage as have the particles that have been on the stage for a long period of time. Consequently, a percentage of the particles travel straight down, or "short circuit", through the stage and thus receive inadequate washing.